1. Field of the Invention
The invention relates to a thermal-insulating material having an essentially magnetoplumbitic crystal structure, a metal substrate having an adhesive layer and a thermal-insulating layer—made of the thermal-insulating material—on its surface, as well as a process for coating of metal substrates with the thermal-insulating material.
2. Description of the Related Art
Thermal-insulating layers are primarily used in stationary gas turbines for energy production and in flight gas turbines. These applications pose more and more increasing demands on the mechanical and thermal properties of the materials to be used. The primary goal of this development is the rise of the temperature of the hot gas, in order to increase the efficacy and the power, when concurrently reducing the fuel consumption. Additionally, it should be achieved, that by a complete combustion of the fuel at high temperatures a lower environmental impact caused by pollutants is allowed.
In order to meet the requests for higher degrees of effectiveness and thus for lower energy consumption, the inlet temperatures of the turbine gas have to be increased to 1500° C. in the future. In this case, the extremely stressed constructional units are the rotating turbine buckets which generally are made of superalloys of nickel and cobalt. Since these alloys have a melting point Ts between 1250° C. and 1320° C. and the surface temperature is ca. 1000° C. to 1200° C. during application, these materials are already used up to 0.9 Ts. The lifetime of these materials was increased by introducing the directionally solidified and monocrystalline turbine buckets and the development of the increasingly expensive film cooling and internal cooling of the turbine elements. However, this causes power losses by the uneconomical increase of the amount of cooling air. Therewith, the limits of the material stress are reached which do not allow a further increase of the gas temperature with usual basis materials.
Another possibility to increase the temperature of the hot gas is the coating of thermally stressed turbine elements with a thermally insulating, ceramic layer of a thermal-insulating material. By application of a ceramic cover layer, the difference between the gas temperature and the temperature of the turbine material can be further increased when using an unchanging amount of cooling air. In order to realize the operation under higher gas temperatures, the basic demands, as a high melting point, a low vapor pressure, a high phase stability, a low thermal conductivity, a possibly high coefficient of thermal expansion, a possibly low density, an adapted emissivity for infrared radiation as well as a high resistance to thermal shock, oxidation and corrosion, must be posed on the material thereby used. All these demands taken together are not met at present by any ceramic material. The partly stabilized zirconium oxide turned out to be the most promising material. The attractiveness of zirconium oxide as a thermal-insulating layer is based upon its coefficient of thermal expansion (11 to 13×10−6 K−1) which is relatively high for ceramic materials, the good thermal shock resistance and the very low thermal conductivity 2 to 3 W/mk).
The present state of the art for producing ceramic thermal-insulating layers are the so called Duplex Systems which consist in general of two different layers. To the layer system are belonging a ceramic cover layer on the surface and a metallic adhesive layer (a so called Bond-Coat—BC) to improve the adhesion of the applied ceramic layer und to further protect the basis alloy from oxidation and hot gas corrosion. Layers of adhesive agents are made of an alloy of M-CrAlY and/or of aluminide. During use and already during the preparation of the layers a thin continuous layer of aluminum oxide is formed on the surface of the bond-coats, which—due to the low oxygen diffusion—represents an effective protection against oxidation for the metallic substrate, and which imparts a good adhesion to the following ceramic layer. Therefore, a complex profile of constraints is posed on the adhesive layer. The adhesive layer has to diminish the internal stresses occurred due to the different thermal expansion coefficients, it has to reduce stress peaks by ductile material behavior and it has to form a dense oxide layer, whereby the growing oxide has to be thermomechanically compatible with the underlying thermal-insulating material.
The preparation of such Duplex Systems has been allowed by virtue of different processes, principally by plasma spraying (PS), the CVD-process and the electron beam evaporation ((Electron Beam Physical Vapor Deposition(EB-PVD)). With PS-processes a lamellar, plate-like microstructure having a high porosity and microcracks is obtained. This morphology is formed by the deformation and the rapid solidification of molten particles on the comparatively cold substrate. Accordingly, the surface of the PS-ceramic layer is rough, and an aftertreatment is necessary, to avoid velocity losses. A further disadvantage of the process is the plugging of boreholes for cooling air during coating, which require another aftertreatment. The advantages of the process are the cost saving and the high degree of automatization.
Less cost saving processes are available in form of high vacuum evaporation techniques which result in qualitatively better layers. The thermal-insulating layers made by EB-PVD possess a columnar microstructure having a high porosity, too. Due to a better expansion tolerance of this columnar morphology, the EB-PVD layers have a longer lifetime, because this morphology effects an extraordinary adaptability of the layer to the mechanical deformation and the thermal expansion of the metallic substrate.
In the EB-PVD process, a finely focused electron beam having programmable pattern is passed in a vacuum chamber over the ceramic to be evaporated which is continuously recharged from below. Depending on the ceramic, a melt having a depth of 1 to 30 mm is obtained which is overheated for a more effective evaporation. The preheated elements or specimens, for instance, turbine elements, are positioned and/or moved in a suitable manner in the vapor cloud, such that by deposition of vapor particles on the surfaces a continuous growing of the layer takes place. Evaporation coating rates of 10–20 μm per minute can be achieved with ZrO2 partly stabilized by Y2O3 (PYSZ) without rotating the sample. In order to achieve stoichiometry with oxide ceramics, the introduction of oxygen is essential in most cases (reactive evaporation coating), for which reason the coating of ceramics is done in a pressure range of between 10−2 and 10−4 mbar.
DE 198 07 163 C1 relates to a thermal-insulating material and processes for the production of the same. A thermochemically stable and phase-stable oxidic thermal-insulating material is mentioned which advantageously can be used in the form of a thermal-insulating layer on thermally highly stressed parts, as for instance, turbine buckets etc. The thermal-insulating material can be processed by plasma spraying, and preferably it consists of a magnetoplumbitic phase in the preferred composition range of MMeAl11O19, whereby M means La or Nd, and Me is chosen from the alkaline earth metals, transition metals and the rare alkaline earth metals, preferably from magnesium, zinc, cobalt, manganese, iron, nickel and chromium.
Magnesium oxide-doped magnetoplumbitic coatings are not suitable for EB-PVD processes. Due to the extreme high vapour pressure of MgO, said oxide precipitates too early from the system and prevents a stoichiometric precipitation. A crystalline precipitation of said phase requires an additional heating, in order to heat die substrate to a temperature above the crystallization temperature (for example >1000° C.).
EP 0 812 930 A1 describes thermal evaporating materials (ingots) for coating articles by physical deposition from the gas phase. In addition to the preparation and use of the same, ceramic evaporating materials are described which comprise a non-sintered mixture of at least two powder fractions of a coarse-grained powder and a fine-grained powder.
EP 0 890 559 A1 describes a process for coating oxidic fibre materials with metal aluminates for the production of failure-resistant, high-temperature stable, oxidatively stable composite materials.
M. K. Cinibulk, J. M. Res., vol. 14, tome 9, pages 3581–3593 (1999), describes the effect of divalent cations in the production of lanthanumhexaaluminate powders and films derived from Citragel. Among other things, magnetoplumbitic materials having the general formula Ln3+M2+1+xQx4+Al11−2xO19 are examined. It has been suggested, to perform fibre coatings with this material.
In spite of the previous success with layers of zirconium oxide, the high-temperature stability remains only partly fulfilled. Thermal stability, i.e. no phase transfer, no change of the Young modulus and the microstructure during application are primary conditions. Furthermore, the weight is contemplated as a critical factor in the design of rotating gas turbines components. Ceramic thermal-insulating layers are not load-bearing materials and add to the weight without increasing the strength. Therefore, such materials which have little weight but which contribute more to the thermal protection, are especially desired.
Essentially, the problem of the invention is based on the preparation of a more stable thermal-insulating material—especially by EB-PVD processes—which has better high-temperatures properties and which is especially suitable for coating of rotating and non-rotating high-temperature stressed elements.